The goal of this project is to develop a prototype catheter- mounted ultrasonic transducer which could be used to direct laser surgery procedures which use light delivered through fiber optic catheters. Laser surgery allows precise ablation of lesions such as tumors, atherosclerotic plaques and vascular malformations at sites which are inaccessible by other techniques. However, it is also relatively easy to damage normal tissues adjacent to or underlying the lesions, or to cause perforations in hollow structures. These complications might be avoided with in situ diagnostic information about the character and thickness of the target tissue and the extent of thermal denaturation caused by the laser energy. Ultrasonic imaging (even simple, one- dimensional) is more likely to provide this information than are optical or radiological techniques. The specific aims of this Phase I proposal are to construct and evaluate a miniature, ultrasonic transducer with a resolution of less than 100 micron and an imaging depth less than 1 cm which could be combined with a side-emitting optical fiber to determine lesion thickness and control ablation. The transducer will be tested in model systems and in animal and human tissues in vitro.